Lithium-ion batteries are characterized, among other things, by a very high specific energy and an extremely low self-discharge.
Lithium-ion cells include at least one positive electrode and at least one negative electrode (cathode and anode, respectively), whereby lithium ions migrate back and forth between the two electrodes during charging or discharging. So-called lithium ion conductors are necessary to allow transport of the lithium ions to take place. In the lithium-ion cells presently used, which are employed, for example, in the consumer sector (mobile telephones, MP3 players, etc.) or as an energy store in electric or hybrid vehicles, the lithium ion conductor is a liquid electrolyte which frequently contains the lithium conducting salt lithium hexafluorophosphate (LiPF6 dissolved in organic solutions. These lithium-ion cells, which include the electrodes, the lithium ion conductor, and current collectors which establish the electrical connections, are enclosed in a package. These packages insulate the battery cells and prevent substances from leaking out of the cell.
One disadvantage of lithium-ion cells containing liquid electrolyte is that with aging and at elevated temperatures, also under thermal stress, the electrolyte component may decompose, resulting in overpressure in the cell. Without appropriate protective measures, this may result in rupture or even combustion of the cells.
Alternatively, it is possible to use a solid ceramic or inorganic lithium ion conductor instead of a liquid electrolyte. As a result of this design, leakage of the electrolyte in the event of damage to the housing is avoided. In addition, it is no longer possible for chemical decomposition to take place when there is a pressure buildup. However, a housing cannot be dispensed with, since the cells must be electrically insulated from one another, and leakage of the active materials of the electrodes must be prevented. This is also the case when multiple battery cells are combined into a single battery housing, since otherwise, internal short circuits may occur between the cells. It is problematic that on the one hand the cells must be correctly connected to one another, for example in a series connection, in order to generate higher voltages, and on the other hand, that the volume of the individual battery cells may change, depending on the state of charge of the battery cell. In addition, it is desirable for the packages of the individual battery cells to take up what may belittle space, since the space required by the cell packages merely enlarges the battery without contributing to the capacity of the battery.
The package is particularly important for battery cells which include corrosive, easily soluble, or volatile components, for example in the area of the cathode, which must not be allowed to penetrate into the area of the anode or also escape from the battery cell. Examples of such include sulfur or polysulfides of a lithium-sulfur battery cell, a sodium-sulfur battery cell, or some other metal sulfide battery cell. Another example is metal fluorides such as iron fluorides (FeF3, FeF2), copper fluorides, or other metal fluorides in a lithium-metal fluoride cell. If the sulfides or fluorides in these examples were to escape from the area of the cathode, this could result in interfering reactions with materials of the anode or other cell components.
The package is likewise particularly problematic from a mechanical standpoint in conjunction with the use of solid materials as electrolyte, separator, and/or ion conductor, since a seal at the solid surfaces or edges reacts sensitively to changes in volume.